The present invention relates to a magnetometer device for measuring weak magnetic fields.
A known magnetometer device for measuring weak magnetic fields from at least one field source comprises a Dewar vessel that defines an inner region. Access to the inner region is through a neck. The inner region contains several superconducting gradiometers as well as their associated SQUIDs. The gradiometers and SQUIDs are cooled by a refrigerant that is fed through the neck from outside the Dewar vessel. This type of magnetometer device is suggested in the publication "Physics Today" of March 1986 from pages 36-44.
Very weak magnetic fields can be measured with superconducting quantum interferometers, sometimes referred to by the acronym "SQUID" which stands for Superconducting Quantum Interference Device. Examples of SQUIDs can be found in, for example, "J. Phys. E.: Sci. Instrum.", Vol. 13, 1980, pages 801-813, or "IEEE Trans. Electron Dev.", Vol. ED-27, No. 10, October 1980, pages 1896-1908. One preferred application for these interferometers is in the area of medical diagnostics. For example, magnetocardiography and magnetoencephalography involve measuring the magnetic fields of heart and brain waves. These waves have magnetic field strengths with magnitudes near 50 pT and 0.1 pT, respectively. See, for example "Biomagnetism-Proceedings Third International Workshop On Biomagnetism, Berlin, 1980", Berlin, N.Y. 1981, pages 3-31. It should be possible to detect these very weak fields in the presence of relatively large interference fields.
Devices are known for measuring such biomagnetic fields. See for example, German Patent No. 3,247,543, corresponding to U.S. Pat. No. 4,749,946. These devices can be designed with one or more channels. The devices contain SQUID magnetometers with gradiometers of the first or higher order, depending on the number of channels as discussed in the "Physics Today" article reference above.
The superconducting gradiometers and their associated SQUIDs are arranged within a Dewar vessel. When cooling the superconducting components, particularly the highly sensitive SQUIDS, it is necessary to avoid direct and indirect magnetic interference. Cooling techniques that use refrigerators have not been favored. Refrigerators generate magnetic fields that cause interference and have moving parts that cause vibrations. These magnetic fields preclude the use of refrigerators. Cooling is thus done by supplying refrigerant such as liquid helium. The refrigerant must be replenished at regular time intervals of several days.
The Dewar vessels containing the superconducting gradiometers and SQUIDs in known magnetometers are arranged above the detectable field sources of the patient to be examined. A Dewar vessel for a multichannel magnetometer device typically contains 15 to 30 liters of liquid refrigerant that ends up suspended directly above the patient. All this refrigerant poses potential danger for the patient located directly underneath. It is possible that a considerable quantity of cold gas would pour over the patient in response to a sudden breakdown of the insulating vacuum or if a sudden damage to a Dewar vessel trips the safety valves. For example, 15 to 30 liters of liquid helium would generate about 10 to 30 cubic meters of cold helium gas in such an emergency. This cold gas is difficult to keep away from the patient, especially if the examination is carried out in a shielded enclosure. An overhead arrangement of Dewar vessels can also be perceived as disagreeable by the patient. Thus there is a need to provide a magnetometer device of this type in which the Dewar vessel is arranged without potential danger to the patient while still not using refrigerators that cause magnetic interference.